linux-kernel - Re: [PATCH 2/2] cpufreq: Update CPU capacity reduction in store_scaling_max

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Date:   Mon, 10 Oct 2022 11:46:29 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        linux-kernel@...r.kernel.org, rafael@...nel.org,
        linux-pm@...r.kernel.org, Dietmar.Eggemann@....com,
        "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>
Subject: Re: [PATCH 2/2] cpufreq: Update CPU capacity reduction in
 store_scaling_max_freq()

+CC Daniel, since I have mentioned a few times DTPM

On 10/10/22 11:25, Peter Zijlstra wrote:
> On Mon, Oct 10, 2022 at 11:12:06AM +0100, Lukasz Luba wrote:
>> BTW, those Android user space max freq requests are not that long,
>> mostly due to camera capturing (you can see a few in this file,
>> e.g. [1]).
> 
> It does what now ?!? Why is Android using this *at*all* ?

It tries to balance the power budget, before bad things happen
randomly (throttling different devices w/o a good context what's
going on). Please keep in mind that we have ~3 Watts total power
budget in a phone, while several devices might be suddenly used:
1. big CPU with max power ~3-3.5 Watts (and we have 2 cores on pixel6)
2. GPU with max power ~6Watts (normally ~1-2Watts when lightly used)
3. ISP (Image Signal Processor) up to ~2Watts
4. DSP also up to 1-2Watts

We don't have currently a good mechanism which could be aware
of the total power/thermal budget and relations between those
devices. Vendors and OEMs run experiments on devices and profile
them to work more predictable in those 'important to users' scenarios.

AFAIK Daniel Lescano is trying to help with this new interface
for PowerCap: DTMP. It might be use as a new interface for those known
scenarios like the camera snapshot. But that interface is on the list
that I have also mentioned - it's missing the notification mechanism
for the scheduler reduced capacity due to user-space new scenario.